All the species of all the three sections are single-brooded. Although the flies themselves can inflict no immediate pain, at their mere sight all the animals out at grass on the farm are seized with an instinctive terror, conspicuously greater than when attacked and copiously bled by any "blinden" breeze flies, which, however, fly more silently and settle on their victims very furtively. One can understand the violent efforts of the horse to free himself from the exceedingly painful bites of a newly attached forest-fly, but one can only wonder at the frantic galloping of oxen and horses to and fro when a non-biting œstrid fly buzzes about like a harmless fat bumble bee and slowly approaches. The females of all the worble-flies, the nostril-flies, and the bot-fly are short-lived, appearing on the wing in August, possibly seen a few days earlier. In the act of ovipositing they make themselves very conspicuous; they lay their eggs whilst hovering in the air, their extruded ovipositors attaching glutinous eggs to their victims. The hatching of the eggs of the bot-fly is assisted by the habit of animals to lick themselves and each other, when certainly their warm, moist tongues will convey into their mouths the newly emerged bot-fly's maggots, which many months later are to be found attached to the internal lining of the unwilling host's stomach. When fully grown in June, these maggots loosen their hold, are discharged with the dung, and pupate in the soil. No satisfactory account has yet been given as to the early stages of the maggots of the worble-flies. The eggs, having been attached to hairs on the host's hide in August, the prominent round pustulent swellings, called worbles, wherein the maggots dwell, do not become conspicuous until the following months of April and May. It is a reasonable surmise that the obscure and long first-period of the maggot's existence may more or less conform to that of some of those flies which are also single-brooded but are predaceous or parasitic on insects. The newly hatched maggot perhaps can crawl, but does not feed until after several moults; at each moulting the strange creature becomes smaller and smaller, but probably at the same time is provided with a new head well suited for the purpose of that period; firstly, with a burrowing or grappling head, and in due time with a feeding suctorial mouth, and then only does practical growth begin. No dipterid flies, at all events, known to be native to Great Britain, possess skin-piercing ovipositors. I have been astonished to read in current literature much about œstrid flies which is not in agreement with my long course of personal observation; for instance, one high authority (F. R. S.) writes that œstrid "_flies_" appear from May until October, and hints that their egg-laying aggressions upon their victims are not conspicuously observable. I feel confident that the facts are quite otherwise.